The invention relates to an infusion device for medical fluids, in particular for an extracorporeal blood circuit.
Specifically, though not exclusively, the invention can be usefully applied for infusing an anticoagulant in an extracorporeal circuit operatively associated with a machine for extracorporeal blood treatment.
Extracorporeal treatments usually include a removal of blood from the patient, external treatment thereof away from the human body, followed by its return, after treatment, into circulation.
Extracorporeal blood is made to circulate through a circuit comprising, in general, an arterial line, or blood removal line, which takes the blood from the patient to a blood treatment device (for example a dialyzer filter) and a venous line, or blood return line, which returns the treated blood to the patient.
To reduce the risk of coagulation of the extracorporeal blood, a known method includes infusion of an anticoagulant (for example heparin) into the extracorporeal circuit, generally into the arterial line, through an infusion line, with relatively low infusion flow rates.
An infusion device which is typically used in this method is a syringe pump, wherein a pushing element, on command of a linear actuator, pushes the plunger of a syringe containing the anticoagulant at an advancement rate which is predetermined and relatively slow. For example, in a dialysis treatment, usually the syringe contains the quantity of anticoagulant necessary for several hours of treatment. The pushing element and the actuator are part of the extracorporeal treatment machine (for example the dialysis machine), while the syringe is generally of the single-use type, or in any case is of the disposable type.
The prior art also teaches an infusion device for a security system against occurrence of overpressures internally of the syringe, due for example to occlusions in the infusion line, with a consequent interruption in the infusion flow.
The delay between the occurrence of an occlusion and its signalling, for example by an acoustic alarm or other form of alarm, is relevant in terms of patient safety. Clinical practice teaches that a loss of anticoagulant infusion for more than fifteen minutes can cause the formation of blood clots in the extracorporeal circuit which, if not quickly identified, can become nuclei of bigger and progressively-growing clots.
A safety system is described, for example, in EP 0 319 648, in which a syringe pump has a sensor for measuring the force acting on the syringe plunger, or on the frontal part of the syringe, and a calculator determines the pressure in the syringe, based on the force measured and the plunger area, previously entered in the calculator, and issues an alarm signal if a predefined maximum pressure threshold is exceeded.
In a further example, U.S. Pat. No. 5,295,967 describes a syringe pump comprising a force transducer for continuously monitoring the force on the syringe plunger, a microprocessor for converting the measured force into a syringe pressure reading, and a display on which the syringe pressure continuously appears in order for the syringe pressure to be monitored during pumping, and for any risk of occlusion to be quickly spotted.
U.S. Pat. No. 5,242,408 describes a security system against the risk of occlusion in a syringe pump, wherein the syringe pressure is calculated by means of a special algorithm, independently of any data regarding the transversal section of the syringe.
The prior art monitoring systems all however exhibit a drawback: they are not able, in good time, to signal a risk of occlusion in the infusion line when the infusion flow rate is very low, as happens for example in an infusion device of an anticoagulant in an extracorporeal circuit.
The above-cited security systems are based, essentially, on the fact that an occlusion along the infusion line leads to an increase in the pressure internally of the syringe. These known systems perform a monitoring of the pressure, emitting an alarm signal when a limit value is exceeded. However, in the case of low infusion flow rates of anticoagulant and/or use of syringes of large dimensions, an occlusion leads to a relatively slow change in the internal pressure of the syringe, and thus to a considerable delay in the alarm signal.
A further cause of delay in alarm signalling is the presence of a low pressure internally of the extracorporeal system, which tends to increase times for reaching the threshold conditions which determine an alarm signalling.